J. Am. Chem. SOC.1994,116, 9407-9408
Generation of the First Persistent Phosphirenylium Cation
9407
Scheme 1
)+
or
Kenneth K. Laali,'J Bernhard Geissler? Oliver Wagner,' Jtirgen Hoffmann,' Ralph Armbrust,' Wolfgang Eisfeld,' and Manfred Regitz'J
1A
Department of Chemistry, Kent State University Kent, Ohio 44242 Fachbereich Chemie, University of Kaiserslautern
Erwin-Schradinger-Strasse 0-67663 Kaiserslautern, Germany
10
2A
2 c
20
Scheme 2
Received June 23. 1994
First synthesized by Breslow in the late 1950s: the cyclopropenium cation, 1, has occupied a central position in carbocation chemistry as the lowest energy form of C3H3+ and the smallest ( 2 ~ Hiickel ) aromatic cationa6 NMR studies,'.* theory,) mass spectrometric studies? and X-ray analysis'O have underscored the importance of the mesomeric 1B form of the cation (Scheme 1). Whereas a number of cyclopropenium cations have been prepared and characterized as stable salts including the parent cation,618 the corresponding phosphirenylium cations, the phosphorus analogues of cyclopropeniumions, have remained hitherto elusive. Thus the relative importance of mesomeric forms 2A and 2B and the potential aromaticity of the system (form 2C) remained open questions (Scheme 1). Parent 2 is the lowest energy C2H2P+ isomer at the HF/631G* level.ll Its C-substituted derivatives had been observed in the E1 mass spectra of 7 and 12 (Scheme 4) and other phosphaalkyne cyclooligomers,12 as well as by collisional decomposition of phosphonium ions derived from tetra-tertbutyltetraphosphacubane and 1,2,5,6-tetraphosphatricyclo[4.2.0.02.5]octadiene.1' In principle, ionization of 7 under suitable conditions could provide access to 2. The synthesis of 7 was achieved by Regitz et al. according to Scheme 2.14 Low-temperature reaction of 7 with AgPF6 gave the fluorophosphirene 9,l5 and the room temperature reaction of 7 with sodium tetraphenylborate (Scheme 3) gave ll.16J7 In an attempt to prevent nucleophile transfer and quenching, ionization of 7 with silver triflate was tried (Scheme 4); instead of 13, the covalent 1-trifloxy-lH-phosphirene 12 was isolated (NMR data in Table 1). There is a consistent trend of deshielding at phosphorus in the C1- F OTf sequence, pointing to increased ionic character of the P-X bond (the '1P NMR resonance for 7 is at -73.4 ppm;'4
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(1) Kent State University. (2) Visiting postdoctoral fellow from Kaiserslautern at KSU. (3) (a) University of Kaiserslautem. (b) OrganophosphorusCompounds. 90. Part 89: Slanv. M.:Renitz. M.Svnthesis. in the ureas. (4) Breslow, R.*j.Am. Chm. Soc. i957,79,5318. Breslow, R.; Yuan, C. J. Am. Chem. SOC.1958,80, 5991. (5) Radom, L.; Hariharan, P. C.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1976, 98, 10. (6) Garratt, P. J. Aromaricity; Wiley: New York, 1986; pp 137-141. (7) Breslow, R.;Groves, J. T.; Ryan,G. J . Am. Chem. Soc. 1967,89,5048. (8) Ciabattoni, J.; Nathan, E. C., 111. J . Am. Chem. Soc. 1%9,91,4766. (9) Lossing, F. P. Con. J. Chem. 1972,50, 3973. (10) Sunderlingham, M.;Jensen, L. H. J . Am. Chem. Soc. 1966,88,198. (11) Maclagen, G. A. R. Chem. Phys. Lett. 1989, 163, 349. (12) Regitz, M.;Wagner, 0. Unpublished results. (13) Laali, K. K.; Geissler, B.; Regitz, M.; Wesdemiotis, C.; Polce, M.; Beranova, S.J. Org. Chem., to be submitted. (14) Wagner,O.;Ehle,M.;Regitz,M.Angew. Chem., In?. Ed. Engl. 1989, 28, 225. Ehle, M.; Birkel, M.; Hoffmann, J.; Regitz, M.Chem. (15) Wagner, 0.; Eer. 1991, 124, 1207. (16) Hoffmann, J. Ph.D. Dissertation, University of Kaiserslautern, 1990.
phbp-cl
pp]
t Bu
11,31-CI-
t Bu
6
7
Scheme 3
7
S
'
10
11
Scheme 4 Ph C A~OSOICFI, Et10 / n C&r - 70 -25 OC
ph)pp-cI
- AgCI
t Bu
P -OSO2CF3
c
7
13
those for 9 and 12 are at -38.2 and -8.9 ppm, respectively's ). 3'PNMRstudieson 12asafunctionofsolvent polarity'gsuggested partial positive charge development at phosphorus and involvement of phosphirenylium cati0n.1~Triflate 12 was trapped with N,Ndimethylaniline.12 The ambient 1 7 0 NMR spectrum of 12 gives just one resonance at 159.2 ppm for the triflate group (no lJop coupling), implying dynamic averaging by dissociation/re"bination.19
I
1.:
/a
.
(d, Jpc= 35.5 Hz) 1 1 5 . 1 .171
